Copies of the letter, signed by the CPJ's executive director, Joel Simon, have also gone to deputy PM, Nick Clegg, Labour leader Ed Miliband, home secretary Theresa May, foreign secretary William Hague and the chairman of the home affairs select committee Keith Vaz.

Here it is in full:

Dear Prime Minister Cameron,

The Committee to Protect Journalists, an international media freedom organisation, calls on you to launch a thorough and transparent investigation into the detention and harassment of David Miranda by the London Metropolitan police and to ensure that his confiscated equipment and data are returned at once.

The use of anti-terror laws to seize journalistic material from Miranda, partner and assistant to Guardian reporter Glenn Greenwald, is deeply troubling and not in keeping with the UK's historic commitment to press freedom.

As reported by The Guardian and other media outlets, Miranda was transiting through London en route from Berlin to Rio de Janeiro when British police stopped and detained him at 8.05 on Sunday at Heathrow international airport.

Miranda has been assisting Greenwald in his reporting, which over the past three months has focused on state surveillance on the basis of documents leaked to Greenwald and the US filmmaker Laura Poitras by former National Security Agency contractor Edward Snowden.

Miranda was acting as a courier of materials between Greenwald and Poitras, The Guardian said, because electronic communications between the two had become insecure in the wake of the Snowden leaks. The Guardian said it paid for Miranda's flight.

As has been widely reported, police held Miranda for the maximum nine hours allowed by the Schedule 7 of Britain's Terrorism Act 2000. They aggressively questioned him about the Guardian's work on the Snowden files, without providing access to a lawyer until the last hour.

They threatened to send him to prison and confiscated all of his electronic equipment, including laptop and hard drive, smart phone, smart watch, memory sticks, DVDs, and a games console.

The electronic equipment, which contained information of journalistic interest, has not been returned, nor did the detaining officers inform Miranda when they would be returning it. The officers, Miranda told The Guardian, coerced him into surrendering the passwords to his computer and phone by threatening him with jail if he did not comply.

Schedule 7 of Britain's Terrorism Act 2000 gives police broad authority to detain, search, and question persons traveling through UK airports in order to determine their possible involvement in terrorism.

Schedule 7 has been widely criticised for allowing police to stop people without suspicion that they have committed a crime, and the UK government is reviewing aspects of the legislation, according to news reports.

It is clear that the police officers who questioned Miranda did not suspect him of terrorism, as they focused their interrogation on Greenwald's, Poitras's, and The Guardian's reporting on state surveillance programs.

Rather, it appears they abused the law to circumvent routine safeguards of the confidentiality of sources and to obtain access to journalistic material. The U.S. has confirmed that it was notified of Miranda's detention, which suggests a coordinated effort.

Miranda's detention is the latest example in a disturbing record of official harassment of The Guardian over its coverage of the Snowden leaks. As Guardian editor-in-chief Alan Rusbridger wrote in a column on Monday, the newsroom has been subjected to government pressure since June to surrender the Snowden-leaked materials in its possession or to destroy them.

On July 20, a day that Rusbridger called "one of the more bizarre moments in The Guardian's long history," two security agents from government communications headquarters looked on as journalists destroyed newsroom hard drives, even though Rusbridger had pointed out that the information existed outside the country.

We call on your government to explain the detention and aggressive interrogation of Miranda; publicly clear him of any connection to terrorist activity; and return his seized equipment as well as any copies made of its contents.

Taking these steps would counter the unsettling perception that the United Kingdom has abused its anti-terrorism laws to impede legitimate journalistic activity carried out in the public interest.

Sincerely, Joel Simon, Executive Director

Norwegian editors and Danish newspaper support The Guardian

The Association of Norwegian Editors have offered their support to The Guardian in a letter to Alan Rusbridger from its assistant general secretary, Arne Jensen.

It says: "We are deeply concerned about what has happened, and we want to share your effort to warn the international media world about the implications of this threat to journalism."

Jensen says the incidents have prompted the association to invite Rusbridger to speak at a conference "to share with us The Guardian's experiences with authorities that try to prevent journalists from doing their job."

And the Danish newspaper, Politiken, has also written to register its "upset after reading about your encounter with the British police and authorities."

Its letter, by international editor Michael Jarlner, says:

"We consider it an attack on the entire press freedom, which must necessarily raise concerns not only in the UK, but also throughout Europe (and the US)."

Politiken would like to see a co-ordinated response by papers across Europe.

But some British journalists (and newspapers) see it differently…

With The Guardian's journalism under attack from both the British and American governments, we might have expected the rest of Fleet Street to rally in defence of press freedom.

Not so, however. The deafening silence I have referred to over the last two days, here and here, has continued today.

There has been sparse coverage of the story and an absence of supportive editorial comment.

Worse, in several references to the disgraceful Miranda detention and astonishing government-ordered destruction of hard drives, The Guardian has come under attack.

Most of the commentators at the Daily Telegraph - Dan Hodges, Louise Mensch, Tim Stanley - have been negative. But today's Telegraph includes an article by Tory MP Dominic Raabe that redresses the balance - and the trio would do well to read it.

"It is an abundant disgrace that British police officers detained and interrogated a foreign national they had not the slightest cause to suspect of any offence, let alone terrorism, in this predictably crude and cretinous manner."